1. Field of the Invention
The present invention relates to a method and apparatus for determining a state of deterioration of three-way reducing and oxidizing catalysts in a double air-fuel ratio sensor system having air-fuel ratio sensors upstream and downstream of the catalysts in an exhaust gas passage.
2. Description of the Related Art
Generally, in a feedback control of the air-fuel ratio sensor (O.sub.2 sensor) system, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and detected engine speed, and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio.
According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratios around the stoichiometric ratio required for three-way reducing and oxidizing catalysts (catalyst converter) which can remove three pollutants CO, HC, and NO.sub.x simultaneously from the exhaust gas.
In the above-mentioned O.sub.2 sensor system where the O.sub.2 sensor is disposed at a location near the concentration portion of an exhaust manifold, i.e., upstream of the catalyst converter, the accuracy of the controlled air-fuel ratio is affected by individual differences in the characteristics of the parts of the engine, such as the O.sub.2 sensor, the fuel injection valves, the exhaust gas recirculation (EGR) valve, the valve lifters, individual changes due to the aging of these parts, environmental changes, and the like. That is, if the characteristics of the O.sub.2 sensor fluctuate, or if the uniformity of the exhaust gas fluctuates, the accuracy of the air-fuel ratio feedback correction amount FAF is also fluctuated, thereby causing fluctuations in the controlled air-fuel ratio.
To compensate for the fluctuation of the controlled air-fuel ratio, double O.sub.2 sensor systems have been suggested (see: U.S. Pat. No. 4,739,614). In a double O.sub.2 sensor system, another O.sub.2 sensor is provided downstream of the catalyst converter, and thus an air-fuel ratio control operation is carried out by the downstream O.sub.2 sensor in addition to an air-fuel ratio control operation carried out by the upstream O.sub.2 sensor. In the double O.sub.2 sensor system, although the downstream O.sub.2 sensor has lower response speed characteristics when compared with the upstream O.sub.2 sensor, the downstream-side O.sub.2 sensor has an advantage in that the output fluctuation characteristics are small when compared with those of the upstream O.sub.2 sensor, for the following reasons.
(1) On the downstream side of the catalyst converter, the temperature of the exhaust gas is low, so that the downstream O.sub.2 sensor is not affected by a high temperature exhaust gas.
(2) On the downstream side of the catalyst converter, although various kinds of pollutants are trapped in the catalyst converter, these pollutants have little affect on the downstream O.sub.2 sensor.
(3) On the downstream side of the catalyst converter, the exhaust gas is mixed so that the concentration of oxygen in the exhaust gas is approximately in an equilibrium state.
Therefore, according to the double O.sub.2 sensor system, the fluctuation of the output of the upstream-side O.sub.2 sensor is compensated by a feedback control using the output of the downstream O.sub.2 sensor. Actually, as illustrated in FIG. 1, in the worst case, the deterioration of the output characteristics of the O.sub.2 sensor in a single O.sub.2 sensor system directly effects deterioration in the emission characteristics. On the other hand, in a double O.sub.2 sensor system, even when the output characteristics of the upstream-side O.sub.2 sensor are deteriorated, the emission characteristics are not deteriorated. That is, in a double O.sub.2 sensor system, even if only the output characteristics of the downstream O.sub.2 are stable, good emission characteristics are still obtained.
Catalysts in a three-way catalyst converter suffer little deterioration if the catalysts are used only in a conventional state, but if leaded gasoline is erroneously introduced into the engine or a high-tension cord is broken or comes loose, to cause misfiring, the catalysts are greatly deteriorated.
In a single O.sub.2 sensor system having an O.sub.2 sensor upstream of the catalysts, even when the catalysts are deteriorated and the pollutant emissions increased, the air-fuel ratio feedback control by the upstream O.sub.2 sensor per se is not affected by this deterioration of the catalysts. Conversely, in the above-mentioned double O.sub.2 sensor system, when the catalysts are deteriorated, the air-fuel ratio feedback control is affected thereby; i.e., when the catalysts are deteriorated and unburned gas such as HC, CO, and H.sub.2 components is allowed to pass therethrough, the output characteristics of the downstream O.sub.2 sensor are also deteriorated and thus the air-fuel ratio feedback control by the downstream O.sub.2 sensor is fluctuated, thereby reducing the emission characteristics, the fuel consumption characteristics, the drivability characteristics, and the like.
Therefore, particularly, in the double O.sub.2 sensor system, it is very important to determine whether or not the catalysts are deteriorated.
In a prior art method of determining a deterioration of the catalysts, the period (frequency) of the output of the downstream O.sub.2 sensor is compared with the period (frequency) of the output of the upstream O.sub.2 sensor, and as a result, when the former is close to the latter, the catalysts are determined to have deteriorated (see: FIGS. 8A, 8B, 8C, 14, 15A, 15B, 15C, 15D, and 16 of U.S. Pat. No. 4,739,614). In another prior art method, a condition of a deterioration of the catalysts is obtained by determining whether or not the number of reversions of the output of the downstream O.sub.2 sensor per unit time is larger than a predetermined number (see: Kokai (Unexamined Japanese Patent Publication No. 63-97852).
In the above-mentioned determination methods, however, the determination is carried out during an air-fuel ratio feedback control by the two O.sub.2 sensors, the fluctuation of the output of the downstream O.sub.2 sensor due to the air-fuel ratio feedback control is superimposed onto the fluctuation due to the deterioration thereof, and accordingly, it is difficult to extract only the fluctuation of the output of the downstream O.sub.2 sensor due to the deterioration thereof. Also, when comparing the output period of the downstream O.sub.2 sensor with that of the upstream O.sub.2 sensor, since the former period is of the order of 1 min and the latter period is of the order of 1 s, it is possible to determined only a completely burned state, or a state close thereto, of the catalysts.
Note that, in a single O.sub.2 sensor system, it is not possible to determine a state of deterioration of the catalysts per se.